Antennas have been long used as devices for converting a high-frequency current into an electromagnetic ray and an electromagnetic ray into a high-frequency current. The antennas are categorized into subgroups such as linear antennas, planar antennas, and solid antennas, based on their shapes. The linear antennas are further categorized into subgroups such as a dipole antenna, a monopole antenna, and a loop antenna, based on their structures. Out of these linear antennas, the dipole antenna, which is disclosed in for example Non Patent Literature 1, is a linear antenna that has a very simple structure, and is widely used as a base-station antenna etc. to this day.
Meanwhile, a terrestrial digital broadcasting service using a terrestrial UHF band (470 MHz to 770 MHz) started on Dec. 1, 2003 in three major wide areas: Kanto, Kinki and Chukyo regions. As analog broadcasting will be terminated in July 2011, the terrestrial digital broadcasting will be capable of providing not only high-resolution digital television programs with high-quality images and sounds but also two-way programs. The terrestrial digital television broadcasting can be received by a UHF antenna, and allows for watching television programs clearly without flickers even on a television set installed in a running train or a running bus etc. Further, services which allow for receiving and watching moving images, data broadcasting and sound broadcasting etc. on a portable information terminal or the like are expected to be provided.
Note here that, as a receiving antenna for a portable device, generally, a rod-shaped monopole antenna is used. The monopole antenna needs to have only one half (i.e., λ/4) the length of a dipole antenna, and therefore can be configured to be relatively small. The monopole antenna requires a conductor plate having an infinite area in theory; however, in a portable device, a conductor plate having a very small area is used as a substitute. Such a monopole antenna for a portable device is also called a “rod antenna” or a “whip antenna”. According to the rod antenna and the whip antenna, a radiation electric field on a top surface of the conductor plate has the same directivity as that of the dipole antenna.
As an antenna for use in a television receiver or a radio receiver for a small portable device, there has been widely known a rod antenna having an extendable structure. The rod antenna is useful, because it can exert its functions when extended and it becomes compact when retracted.
As an antenna device using the rod antenna, for example, there has been proposed a device in which (i) a feed pin of a planar antenna is constituted by an extendable rod antenna and (ii) electric connection and disconnection between an extraction conductor of the rod antenna and a patch-shaped conductor of the planar antenna enable the antenna device to serve as a circularly polarized wave antenna and a linearly polarized wave antenna.
Further, there has been known a “helical antenna” as another arrangement example of the rod antenna. The helical antenna is formed by spirally winding an antenna line around a rod. Generally, an antenna using a conducting wire longer than a wavelength has a wide useable band. Therefore, the helical antenna can be downsized while keeping its wide-band characteristic by virtue of its winding structure. Further, the helical antenna serves as a flexible antenna which is tough and flexible (has safety), by constituting the rod (core) by a flexible material.
Such antenna devices for portable devices operate in 470 MHz to 770 MHz, and few of them alone can cover all of the channels of the terrestrial digital broadcasting. Further, in order to realize an antenna device for a portable device which antenna device can cover all of the channels, it is necessary to cause the antenna device to include a tuning circuit to tune the antenna device to a receiving frequency by controlling voltages. The same applies to an antenna for movable bodies, which antenna is to be provided in a movable body such as a vehicle.
Further, since antenna devices for portable devices and antenna devices for movable bodies are incapable of obtaining sufficiently-good radiation characteristics in a whole terrestrial digital broadcast band, most of them support only one-segment broadcasting and few of them support all of the 13 segments. This is because, in order for an antenna device to support all of the 13 segments, the antenna device is required to have an SN ratio (signal-to-noise ratio) higher than that of the antenna device which supports only the one-segment broadcasting.
Note here that the terrestrial digital broadcasting is a broadcasting system in which a 6 MHz domain is divided into 13 segments to carry out transmission. On the other hand, the foregoing “one-segment broadcasting” is a service which allows for partial reception of only one segment in the middle of the 13 segments, which one segment alone carries images, sounds and data for mobile phones and mobile terminals. This service started on Apr. 1, 2006 (Sat). Such a one-segment broadcasting service delivers programs that are basically the same as those delivered via 12 segments for usual television receivers. Therefore, users can watch popular programs that they usually watch on television sets installed in their home, even when they are away from home.
Under such circumstances, if an antenna device for terrestrial digital broadcasting is put into practical use, it is possible to mount such an antenna device not only on a mobile phone but also on various types of receivers such as car navigation systems, personal computers and dedicated portable television sets. This allows for reception of high-quality images as compared to one-segment broadcasting.